1. Field of the invention
The present invention relates to a direct memory access apparatus in an image processing system and an external storage device used therein. More specifically, the present invention relates to a direct memory access apparatus used in an image processing system which displays a image on a raster scan type monitor such as a CRT display or the like.
2. Description of the prior art
One prior art image processing device that displays an image on a raster scan monitor is disclosed in Japanese Patent Publication No. 2-7478 published on Feb. 19, 1990 (corresponding U.S. Pat. No. 4,824,106). When changing display data on a given horizontal line in such a type of image processing device, generally, a CPU verify that the monitor has entered a horizontal blanking period and then the CPU changes either the display data or display parameters.
One disadvantage of the above-noted prior art is that the CPU is overburdened in executing its tasks. Another disadvantage is that because the horizontal blanking period is short, only a limited amount of data can be changed during that period. In TV game machines that do not afford expensive high-speed CPU's, the program involved can be quite complex. This makes it practically impossible to change display data on a horizontal line by horizontal line basis.